Method and apparatus for identifying and analyzing vapor elements

ABSTRACT

A method and apparatus for identifying and analyzing vapor elements, using a preconcentrator collector. The preconcentrator collector collects and preconcentrates chemical vapors to be detected and identified before chromatographic analysis using surface acoustic wave gas chromatograph (SAW/GC) technology. The preconcentrator collector is used in conjunction with a sensor in an SAW/GC detector in the apparatus. A physical parameter associated with the sensor changes in a defined manner upon exposure of the sensor to an unknown vapor, permitting identification of the individual vapor elements. The preconcentrator collector of the invention includes a body portion having an inlet and an outlet, and a stack of collector plates disposed in the body portion. The collector plates are made of a material that is easily micromachinable and easily cleanable, such as silicon, silica or fused quartz. The outlet of the body portion is connectable to a sampling pump for taking a sample of ambient air into the body portion through the inlet. After the sample is taken, the collector plates trap the chemical vapors in the sample of ambient air and non-trapped vapors exit through the outlet. By using the preconcentrator collector, the present invention achieves more specificity and selectivity simultaneously with high sensitivity.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and incorporates by referencepatent application entitled “Method and Apparatus For Identifying AndAnalyzing Vapor Elements” Ser. No. 08/820,671, filed on Mar. 18, 1997,(and now U.S. Pat. No. 5,970,803), by inventors Edward J. Staples andGary Watson.

A claim for priority is hereby made to a U.S. Provisional ApplicationSer. No. 60/013,891, filed on Mar. 22, 1996.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method and apparatus for identifying andanalyzing vapor elements, and more particularly to a surface acousticwave gas chromatographic (SAW/GC) with a preconcentrator collector and amethod of using same.

2. Related Art

In the analysis of chemical vapors there is frequently a need to detectextremely small trace amounts of a specific vapor amongst a backgroundof different chemical species. Examples are the detection of contrabandaboard the cargo of vessels being inspected by the United States CoastGuard and the detection of leaking explosive or dangerous chemicals atdepositories thereof. Automated or portable testing apparatus for thispurpose has generally not been available. Further, existing detectorsare only able to detect chemical species at concentrations well abovetheir ambient vapor concentrations and thus lack sufficient sensitivity.

Chemical sensors have been developed that physically change uponexposure and contain absorbing polymers selected for their affinity toabsorb a group of related chemical species. One type, surface acousticwave (SAW) delay line sensors, are the most developed and readilyavailable. For example, one commercial supplier is Microsensor System,Inc., Fairfax, Va.

A method and apparatus for using an SAW device to detect a vapor isdisclosed by H. Wohltjen in U.S. Pat. No. 4,312,228, issued Jan. 26,1982. As described therein, the SAW device comprises a piezoelectricelement having a surface coated with a polymer material selected toabsorb and react with the chemical to be detected. Interaction of thechemical with the material coating of the sensing element alters one ormore properties of a surface acoustic wave, and the electrodes on thepiezoelectric element detect the altered wave, producing an electricalsignal.

Another apparatus and method for detection and identification ofchemical vapors is disclosed in U.S. Pat. No. 4,895,017. As described ina plurality of surface acoustic wave (SAW) devices, each coated with aselected polymer material, are exposed to the vapor to be analyzed. Inthis invention a predicted time constant (or rate) of diffusion into thepolymer coating is used to identify the different chemical species. Toquantitatively identify specific chemical species present in vapors anarray of SAW sensors with different polymer coatings may be exposed anda pattern recognition technique utilized to identify specific species.This is described in a paper entitled “Correlation of Surface AcousticWave Device Coating Responses With Solubility Properties and ChemicalStructure” by D. S. Ballentine, Jr., S. L. Rose, J. W. Grate, and H.Wohltjen, published in Analytical Chemistry, Vol. 58, p. 3058, December1986.

A further patent using multiple polymer coated dispersive delay lines isdisclosed by J. Haworth in U.S. Pat. No. 5,012,668, issued May 7, 1991.The use of specific absorbant polymers to sensitize the surface of apiezoelectric crystal and induce a phase or amplitude variation in atraveling acoustic wave is common to all of the prior art and thisapproach severely limits the performance of these vapor detectors.Multiple polymer films dilute the vapor samples and thereby limit theamount of vapor that can be detected by each film. Also, practically anytype of film applied to the surface of a piezoelectric crystalintroduces noise which reduces sensitivity further.

In view of such problems, the present inventors have proposed anapparatus for performing high speed detection and identification ofvapor species. The apparatus includes a temperature programmed vaporpreconcentrator for trapping condensable vapor species, a multi-portvalve, a temperature programmed chromatographic capillary column, anacoustic wave interferometer for detecting adsorption and desorption ofvapor species, a thermoelectric heating and cooling element forcontrolling the temperature of the acoustic interferometer sensor, andan electronic system controller which is described by the presentinventors, i.e., Staples et al., in U.S. Pat. No. 5,289,715, which ishereby incorporated by reference. This apparatus is capable of detectingtrace elements with high specificity and sensitivity. The detection canbe done near real time.

SUMMARY OF THE INVENTION

It is an object of the present invention to achieve more specificity andselectivity simultaneously with high sensitivity by providing apreconcentrator collector for preconcentrating chemical vapors to bedetected and identified before chromatographic analysis.

It is another object of the present invention to provide improvedperformance over the conventional surface acoustic wave gaschromatography (SAW/GC) technology.

According to a first aspect of the present invention, a preconcentratorcollector is provided for collecting and preconcentrating chemicalvapors from a sample of ambient air. The preconcentrator collectorcomprises a body having an inlet and an outlet that is connectable to asampling pump for taking the sample of ambient air into the body throughthe inlet. The preconcentrator collector also includes a stack ofcollector plates made of a material that is easily micromachinable andeasily cleanable, such as silicon, silica or fused quartz, and disposedin the body. According to the invention, after the sample is taken, thecollector plates trap the chemical vapors in the sample of ambient airand non-trapped vapors exit through the outlet.

According to a second aspect of the present invention, there is providedan apparatus for identifying and analyzing chemical vapors from a sampleof ambient air. The apparatus comprises a sampling pump, apreconcentrator collector coupled to the sampling pump for collectingand preconcentrating chemical vapors taken from the sample of ambientair; a separating means for separating individual vapor species in thechemical vapors desorbed from the collector plates of thepreconcentrator collector according to their speeds in traveling throughthe separating means; and a detecting means for detecting andidentifying the individual vapor species output from the separatingmeans. The separating means may be made of a metal capillary columnheatable by applying an electric current thereto.

According to a third aspect of the present invention, there is provideda method of detecting and identifying chemical vapors from a sample ofambient air. The method comprises the steps of collecting andpreconcentrating chemical vapors taken from the sample of ambient airusing a preconcentrator collector; separating, using a separating means,individual vapor species in the chemical vapors desorbed from thecollector plates of the preconcentrator collector according to theirspeeds in traveling through the separating means; and detecting andidentifying the individual vapor species output from the separatingmeans using a surface acoustic wave gas chromatographic (SAW/GC)detector.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an SAW/GC system according to an embodiment of the presentinvention;

FIG. 2 shows an exploded view of a hand-held embodiment of an SAW/GCsystem according to the present invention;

FIGS. 3A and 3B illustrate the operation of the preconcentratorcollector in accordance with the present invention;

FIGS. 4A and 4B show the front and back sides of a micromachined siliconcollector plate used in the preconcentrator collector of the presentinvention;

FIG. 5 shows an exploded view of a sampling valve used in thepreconcentrator collector of the present invention;

FIG. 6 shows an exploded view of the preconcentrator collector inaccordance with the present invention;

FIG. 7 shows the fully assembled preconcentrator collector shown in FIG.6; and

FIGS. 8A-8C show the operation of the SAW/GC system with thepreconcentrator collector in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an SAW/GC (surface acoustic wave gas chromatographic)system according to the invention. The SAW/GC system comprises asampling pump 24, a preconcentrator collector 26, a six-port GC valve30, a loop trap section 32, a vacuum pump 34, a capillary GO column 42and a SAW detector 53.

In order to achieve specificity and selectivity simultaneously with highsensitivity, a silicon collector preconcentrator 26 is utilized in apreferred embodiment of the SAW/GC detection system according to theinvention. In gas chromatography, a sample containing many differentchemical species is taken by collector preconcentrator 26 through theuse of a sampling pump 24. The sample then passes through GC valve 30and enters into capillary column 42 together with an inert carrier gassuch as helium. As the chemicals travel along the capillary theyinteract with the chemical coating of capillary column 42 and are slowedby the interaction. Since each interaction is chemically different, theindividual chemical species exit capillary column 42 dispersed in time.By measuring the time to transit capillary column 42, the individualspecies can be identified. By using a nozzle, the effluent fromcapillary column 42 is focused into an area of SAW resonator containingthe highest intensity of standing acoustic waves in SAW detector 53,whereby the chemical species will be detected only if they are adsorbedonto a crystal surface of a sensor in SAW detector 53. An example of SAWdetector and its operation is disclosed in U.S. Pat. No. 5,289,715,which has been incorporated by reference.

By means of a six-port GC valve, ambient air is sampled, aerosolstrapped, and then injected into the helium flow to capillary column 42.In FIG. 1, The SAW/GC system will sample vapor from the ambientenvironment through a silicon or silica preconcentrator collector 26 andsampling pump 24. Preconcentrator collector 26 serves the dual purposeof selectively adsorbing vapors or particles from the air and allowinglarge volumes of air to be sampled while minimizing the volume of thepreconcentrator collector itself. After the vapors or particles areadsorbed, the output of preconcentrator collector 26 is cycled via asampling valve in the preconcentrator collector to allow the vaporgenerated from the thermal desorption at preconcentrator collector 26 toenter loop trap section 32 and stick thereon.

FIG. 2 depicts an exploded view of a hand-held SAW/GC system accordingto the invention. Silicon preconcentrator collector 26, six-port GCvalve 30, capillary column 42, SAW detector 53 of the SAW/GC system areillustrated in FIG. 2. Also illustrated are a housing 62 for containingthe system, a sensor clamp 64 for securing SAW detector 53 in thesystem, and insulation section 66 for heat insulation.

In this embodiment, silicon preconcentrator collector 26 is integratedon the inlet of the system. To minimize power, the new system utilizes adigitally controlled, temperature ramped capillary column 42 which isable to provide a linear ramp to over 200° C. in 5 seconds. Capillarycolumn 42 is made of metal and surrounded by an adhesive stiff plasticpiece 44, such as Kapton made by 3M Company. Stiff plastic piece 44secures capillary column 42 in the system. An air gap 45 is provided toallow air to flow in and out of the system. The system further minimizespower by utilizing a variable duty cycle pulse width modulationtechnique to apply current to metal capillary column 42 to heat it. Theresult is a system which can produce a chromatogram that will last only10 seconds while utilizing minimum power.

A purpose of integrating a front end preconcentrator collector 26 intothe SAW/GC system is to make it possible to test large samples at a rateof 5 to 10 liters of air per minute. The present invention utilizes manysmall nozzles or holes provided on a temperature controlled siliconcollecting surface of preconcentrator collector 26 to adsorb and desorbvapors and particles from ambient air. Passivated silicon (silica)plates are used in preconcentrator collector 26. They are very effectivefor trapping “sticky” materials such as chemical vapors and areresistant to collecting dust and other interference that trouble “wipe”type collection systems that require physical contact with the objectbeing tested.

FIGS. 3A and 3B illustrate the operation of preconcentrator collector 26in accordance with the invention. Objects can be screened by thepreconcentrator collector at distances up to several centimeters withoutactual physical contact between the preconcentrator collector and theobjects. The preconcentrator collector operates in two steps. FIG. 3Aillustrates high volume airflow during sampling. Air above the surfaceof an object 72 to be “smelled” is pulled through a stacked array ofholes in micromachined silicon wafers 76 by a high capacity samplingpump through a sampling pipe 77. Aerosols entrained in the air stick tosilicon wafers 76. Adsorption to the silica surface is enhanced by a lowambient air temperature and can be further enhanced by selectivechemical coatings applied to the front surface of silicon wafers 76.After the adsorption, the temperature of silicon wafers 76 is quicklyraised to desorb the trapped material. The desorbed vapor then entersthe SAW/GC sensor through a transfer pipe 78, as illustrated in FIG. 3B.Attached to the inlet of preconcentrator collector 26 is an annular ring79 with small directional holes. Pulsed air jets from the holes withinthe ring break up boundary layers air along the surface being sampled.

In this embodiment, the use of a puffing technique by employing aseparate pulsed air supply can enhance the collection efficiency byseveral orders of magnitude over what would normally be expected fromvapor pressure predictions.

The preconcentrator collector comprises a stack of silicon or silicacollector plates 76. The front and back sides of one such plate areshown in FIGS. 4A and 4B, respectively. As shown in FIGS. 4A and 4B, thecollector includes a thin silicon or silica membrane 82 on which aplurality of through holes 84 are provided. A thin film resistive heateris attached to the front side of membrane 82. On its back side, aplurality of spacers 88 made of silicon dioxide, for example, arepositioned between adjacent holes 84 across membrane 82 in onedirection. A support ring 90 is provided around membrane 82. Thecollector plates allow air to pass through while trapping any aerosolswithin the air. After the aerosol material has been trapped on thesurface of the collector plates, the temperature of the collector platesis raised rapidly by applying electric current to thin film heater 86 soas to desorb the trapped material as vapor. The vapor then enters theSAW/GC detector as illustrated in FIG. 3B. In effect, each hole 84 in acollector plate acts as a nozzle which directs impingement flow onto thesurface of the plate beneath it.

Collector plates are used in pairs with offsetting through holesmachined into them. Preliminary flow calculations for 0.01 to 0.1 cmdiameter holes have been performed. For example, consider a wafer with100 holes of 0.04 cm diameter. The calculations show that with only atwo psi pressure drop across the wafer, a flow of 59.803 liters/minutecan be achieved. This means that one liter of ambient air can bescreened in one second.

Silicon is an ideal material for a number of reasons. First, it isrelatively inert, particularly when oxidized, and can be easily cleaned.This is important since extraneous vapors are undesirable. Second,silicon can be accurately micromachined into structures with low thermalmass. The collector plates must be heated and cooled quickly with aminimum of applied power. For silicon wafers of 2.5 cm in diameter with100 holes each having 0.025 cm in diameter, a surface area of 9.916 cm²will be available for collecting aerosols. The total mass for a wafer.of 0.0025 cm in thickness will be 0.0987 grams. This is 10 times lessthan the mass of a tubular type preconcentrator with equivalent surfacearea. Preliminary heating requirements as a function of wafer thicknesshave been calculated. The power required to raise the temperature ofthis wafer to 200° C. in 1 second is only 6.053 watts. Energy is smallsince power only needs to be applied only for 1 second and the thermalmass is low. Other materials may also be used instead of silicon to makethe collector plates as long as they can be easily micromachined andeasily cleaned. For example, fused quartz is one such material.

In FIG. 4A, the collecting surface on the front side of the collectorplate may be made of epitaxial silicon on a low resistivity bulk siliconwafer. Anisotropic etching using ethylenediamine, pyrocatechol, andwater (EDP) will create thick supports and a thin (5-10 μmeter) membranewith holes. For a detailed description of this type of etching, refer toR. M. Finne and D. L. Klein, “Etching of Silicon Using EDP”, J.Electrochem. Soc., Vol. 127, No. 12, December 1980; E. J. Staples, “HighResolution SWD Pattern Replication Without Photo Chemical Processing,”Sonics and Ultrasonics Symposium Proceedings, November 1973, pp.522-528; and R. D. Jolly and R. S. Muller, “Miniature Cantilever BeamsFabricated by Anisotropic Etching of Silicon,” J. Electrochem. Soc.,Vol. 127, No. 12, pp. 2751-2754, December 1980, all of which are herebyincorporated by reference. The final process step will involve vacuumdeposition of a thin film resistive heater in the form of a meander lineon the front surface. In one embodiment, wafers of one inch in diameterare used; in other embodiments, wafers of as large as six inches indiameter may be used. Based upon previous desorption experiments (See E.J. Staples, G. W. Watson, and W. J. Horton, “Temperature ProgrammedDesorption Characteristics of SAW Resonators”, Proceedings of 1991Ultrasonics Symposium, pp.317-320, Nov. 19, 1991, which is herebyincorporated by reference), and temperature and energy calculations, thetemperature of the collector plates can be raised above 300° C. Thiswill ensure rapid cleanup of the collecting surfaces of the collectorplates.

Another important element of the preconcentrator collector design is thesampling valve. FIG. 5 shows an exploded view of a poppet-type samplingvalve assembly 100 which may be used in the preconcentrator collector ofthe invention. As shown in FIG. 5, sampling valve assembly 100 comprisesa sealing plate stopper 102 having six pumping ports 104, a sealingplate 106, and a base plate 108 which includes holes 110 and 112. Twopipes 77 and 78 are connected to the internal chamber of thepreconcentrator collector through holes 110 and 112, respectively. Pipe77 is a transfer pipe for transferring the concentrated gas desorbedfrom the collector plates to the SAW/GC detector. Pipe 78 is a samplingpipe for withdrawing ambient gas through the preconcentrator collector.Sampling pipe 78 is connected to a sampling pump. Transfer pipe 77penetrates base plate 108 of sampling valve assembly 110 and sealingplate stopper 102. An opening 116 of transfer pipe 77 is disposed closeto the collector plates without contacting them. On the other hand,sampling pipe 78 has an opening 117 connected to the internal chamber ofthe collector plates through hole 112 at base plate 108. Sealing plate106 is disposed between base plate 108 and sealing plate stopper 102.When the ambient air is sucked through sampling pipe 78, sealing plate106 moves close to base plate 108 and the ambient gas can flow throughsix ports 104 provided in sealing plate stopper 102. When the samplingpump connected to sampling pipe 78 is turned off and the desorbed gas issucked toward the SAW/GC detector, sealing plate 106 moves towardsealing plate stopper 102, closing ports 104 on stopper 102. Then, thedesorbed gas flows into transfer pipe 77 to the SAW/GC detector. Thisoperation is also shown in FIG. 3B.

FIG. 6 shows an exploded view of a complete preconcentrator collectorassembly 26 according to one embodiment of the invention. As shown inFIG. 6, preconcentrator collector assembly 26 comprises a poppet-typesampling valve 100, collector plates 76, body segment 122, a tip or nosecone 124, and an annular puffing ring 79. Body segment 122 and samplingvalve 100 of preconcentrator collector assembly 26 will be heated bymeans of external electrical heaters (not shown). They are surroundedpartially by insulation section 66 shown in FIG. 2 so that heat loss isminimized. Tip or nose cone 124 of preconcentrator collector assembly 26may be fabricated from a machinable ceramic insulating material so thatonly a small amount of heat will pass through it to annular puffing ring79 to heat the gas passing through ring 79. Since ceramic tip 124 andpuffing ring 79 may come in contact with the object or person to beanalyzed, it is important that both should not be too hot to touch. Atemperature of approximately 60° C. is desirable. A completely assembledpreconcentrator collector assembly 26 is shown in FIG. 7.

An example operation of the vapor detection and identification apparatusincorporating the inlet preconcentrator collector according to thepresent invention is illustrated in FIGS. 8A, 8B and 8C. When samplingpump 24 is turned on, the ambient gas begins to flow throughpreconcentrator collector 26. Chemical species to be sampled areadsorbed onto the surface of the collector plates in preconcentratorcollector 26. Then, sampling pump 24 is switched off and the collectorplates begin to be heated by applying electric current to thin filmheater 86 to desorb any trapped materials to vapor. Chromatography valve30 is set in the collection position as shown in FIG. 8A. A sample ofcollector 26 gas atmosphere and any condensable vapors contained thereinis made to pass by means of a vacuum pump 34 through loop trap section32 which is cooled by ambient air flow to a temperature sufficient toadsorb condensable vapor species. Loop trap section 32 is made of metal,such as brass, and condenses vapor species. The apparatus works with anyambient atmospheric gas, although air is the most Common gas to besearched for specific condensable vapor species.

During several seconds of sampling time, a carrier gas supply 50 causeshelium gas to flow through a filter 48 and a flow controller 44, afterwhich the helium gas is passed through chromatography valve 30,capillary separation column 42 and strikes the collection surface of thepiezoelectric crystal sensor (not shown) in acoustic wave interferometer40 in SAW detector 53. The preferred carrier gas is helium although theapparatus will operate with any inert carrier gas. During the initialsample collection cycle, the temperature of the acoustic interferometersensor is raised by heat from a thermoelectric element (not shown) to atemperature of 120° C. sufficient to clean the sensor surface by causingall adsorbed vapors to desorb and pass out through an exhaust vent 54.After the sample collection time has expired, vacuum pump 34 is turnedoff, the temperature of acoustic wave interferometer 40 is reduced to atemperature of 5° C. sufficient to induce adsorption, and chromatographyvalve 30 is switched to the transfer position shown in FIG. 8B. Afterchromatography valve 30 has switched to the transfer position, thehelium carrier gas flows through loop trap section 32, chromatographyvalve 30, capillary separation column 42, and through acoustic waveinterferometer 40 to exhaust vent 54. At this time a short 0.0025 secondpulse of electrical current is made to flow through loop trap section 32which rapidly raises its temperature to 200° C. by means of resistiveheating, and results in desorption of the trapped vapor species into thecarrier gas. The carrier gas carries these desorbed vapors through theheated chromatography valve 32 to capillary separation column 42.Immediately following the heating of loop trap section 32 is acooling-off period of from 1 to 10 seconds, during which the loop trapsection 32 is cooled back to ambient temperature by conduction andradiation into the ambient air.

Following the transfer cycle, chromatography valve 30 is returned to thesample position. The preferred time for the transfer is about onesecond. After the transfer, a third cycle of the injection and analysiscycle is initiated, as illustrated in FIG. 8C. In this cycle, a newinlet sample vapor is condensed onto loop trap section 32, causing theadsorbed vapor species to be rapidly desorbed and injected into thecarrier gas flowing through capillary separation column 42. The rapidlydesorbed vapors are injected into capillary separation column 42 as aninjection burst of vaporous material.

As is well known to practitioners of gas chromatography, individualvapor species passing through a separation column travel at differentspeeds and hence individual vapor species exit the separation column atdifferent times. The preferred embodiment uses a 36 inch length of 0.008inch inside diameter quartz capillary coated with 5% phenyl phase (DB-5)bonded to the backbone silicon atoms of a polysiloxane stationary phasepolymer. The capillary separation column is commercially available fromJ & W Scientific, Folsom, Calif. The preferred embodiment maintains thecapillary column at a temperature of 200° C. and all vapor species exitthe column within a time span of 5 to 10 seconds.

The gas flow passes through the separation column to a nozzle whichcauses the individual vapor species to be focused onto a geometricallyconfined and focused area of the acoustic wave interferometer. Thecondensable vapor species are condensed at the interferometer due to thetemperature gradient between the vapor stream and the collection surfaceat the interferometer. Uncondensed vapors pass on to the exhaust vent.The temperature gradient between the nozzle gas temperature and thepiezoelectric crystal sensor of the interferometer is sufficiently largeso that all of the condensable vapors are collected and focused into anarea of maximum amplitude standing acoustic waves on the piezoelectriccrystal sensor surface.

While the present invention has been described with respect to preferredembodiments and modifications thereto, it will be understood by those ofordinary skill in the art that further modifications may be made withinthe scope of the claims that follow. Accordingly, the scope of theinvention is not to be in any way limited by the disclosure of thepreferred embodiments, but to be determined by reference to the claims.

What is claimed is:
 1. A preconcentrator collector for collecting and preconcentrating chemical vapors from a sample of ambient air, comprising: a body having a sampling pipe and a tranfer pipe, said sampling pipe being connectable to a sampling pump for taking the sample of ambient air into the body; and a stack of disk-like collector plates each having a multiplicity of holes passing normally therethrough disposed in the body; wherein after the sample is taken the collector plates trap the chemical vapors in the sample of ambient air and non-trapped vapors exit through an exhaust.
 2. The preconcentrator collector of claim 1, further comprising a sampling valve disposed within said body.
 3. The preconcentrator collector of claim 2, wherein the sampling valve is a poppet-type sampling valve comprising: a sealing plate stopper having a plurality of pumping ports; a sealing plate; and a base plate; wherein the sealing plate is disposed between said sealing plate stopper and said base plate.
 4. The preconcentrator collector of claim 3, further comprising a puffer attached to the body portion at the inlet and having a plurality of directional holes for taking the sample of ambient air.
 5. The preconcentrator collector of claim 4, wherein the body portion includes a nosecone at the inlet and the puffer is attached to the nosecone.
 6. The preconcentrator collector of claim 4, further comprising a sampling pipe and a transfer pipe, the sampling pipe having a first end coupled to the sealing plate stopper, the sealing plate and the base plate of the sampling valve and having a second end for connecting to a sampling pump, the transfer pipe having a first end coupled to the base plate of the sampling valve and a second end for connecting to a surface acoustic wave gas chromatographic (SAW/GC) detector, wherein in a sample collection cycle, the sample of ambient air is sucked into the preconcentrator collector by the sampling pump, with chemical vapors being trapped on the collector plates, and in an analysis cycle, the chemical vapors trapped by the collector plates are desorbed and transferred to the SAW/GC detector for detection and identification.
 7. The preconcentrator collector of claim 2 further comprising a loop trap section disposed between ports of said sampling valve such that by way of appropriate switching of said sampling valve, said loop trap section is placed in fluid communication with said sampling pipe in a first instance or with said transfer pipe in a second instance, whereby said loop trap section may serve to adsorb chemical species, on the one hand, or to desorb said chemical species for purposes of analysis on the other hand.
 8. A preconcentrator collector for collecting and preconcentrating chemical vapors from a sample of ambient air, comprising; a body having an inlet and an outlet, the outlet being connectable to a sampling pump for taking the sample of ambient air into the body; a stack of collector plates disposed in the body and made of a material selected from the group consisting of silicon, silica and fused quartz, each of the collector plates comprising: a membrane having a plurality of through holes spaced apart from one another, a film heater attached on a surface of the membrane for changing the temperature of the collector plate; and a sampling valve attached to the body; wherein after the sample is taken, the collector plates trap the chemical vapors in the ambient air and non-trapped vapors exit through an exhaust.
 9. The preconcentrator collector of claim 8, wherein the sampling valve is a poppet type sampling valve comprising: sealing plate stopper having a plurality of pumping ports; a sealing plate; and a base plate; wherein the sealing plate is assembled between the sealing plate and the base plate.
 10. An apparatus for identifying and analyzing chemical vapors from a sample of ambient air, comprising: a sampling pump; a preconcentrator collector for collecting and preconcentrating chemical vapors taken from the sample of ambient air, the preconcentrator collector comprising: a body having a sampling pipe and a tranfer pipe, the outlet being coupled to the sampling pump for taking the sample of ambient air into the body, and a stack of disk-like collector plates each having a multiplicity of holes passing normally therethrough disposed in the body, wherein after the sample is taken the collecting plates trap the chemical vapors in the ambient air and non-trapped vapors exit through an exhaust; means for desorbing said chemical vapors trapped on said collecting plates; separating means for separating individual vapor species in the chemical vapors desorbed from the collector plates of the preconcentrator collector according to their speeds in traveling through the separating means; and detecting means for detecting and identifying the individual vapor species output from the separation means.
 11. The apparatus of claim 10, wherein the collector plates are made of a material selected from the group consisting of silicon, silica and fused quartz.
 12. The apparatus of claim 10, wherein each of the collector plates comprises: a membrane having a plurality of through holes spaced apart from one another; a film heater attached on a surface of the membrane for changing the temperature of the collector plate.
 13. The apparatus of claim 12, wherein the collector plates are made of a material selected from the group consisting of silicon, silica and fused quartz.
 14. The apparatus of claim 12, wherein the separating means includes a metal column which is heatable by applying electric current thereto.
 15. The apparatus of claim 10 further comprising a sampling valve attached to the body portion at the outlet.
 16. The apparatus of claim 15 wherein the sampling valve is a poppet type sampling valve comprising: a sealing plate stopper having a plurality of pumping ports; a sealing plate; and a base plate; wherein the sealing plate is assembled between the sealing plate and the base plate.
 17. The apparatus of claim 10 wherein the detecting means includes a surface acoustic wave gas chromatographic (SAW/GC) detector.
 18. The apparatus of claim 10 further comprising: a gas chromatographic (GC) valve coupled between the preconcentrator collector and the separating means for coordinating transfer of chemical vapors from the preconcentrator collector to the detecting means via the separating means.
 19. The apparatus of claim 18, further comprising a container coupled to the GC valve and containing a carrier gas which carries the chemical vapors from the preconcentrator collector to the SAW/GC detector via the separating means.
 20. The apparatus of claim 19, wherein the carrier gas is helium. 